In a sheet-fed, rotary offset printing press, a plate cylinder is fitted with a thin sheet printing plate having the image to be printed photosensitized thereon. The printing plate is supplied with ink and dampening fluid, such that the ink is attracted and dampening fluid is propelled (hydrophobic) to appropriate parts of the photosensitized cylinder and the ink is repelled and the dampening fluid is attracted and coats (hydrophilic) the other parts. This forms an image. Through rolling contact with the blanket cylinder, the image is transferred to the blanket cylinder. The blanket cylinder rolls against an impression cylinder having a sheet of paper or other material to be printed gripped at a leading edge and wrapped around the surface of the impression cylinder. Through rolling contact under an impression force or rolling pressure, the blanket cylinder transfers the ink image onto the paper.
In a multiple color offset printing press, such as a three or four-color printing press, there is a plate cylinder and a blanket cylinder for applying each color with a dot pattern. Each plate for the plate cylinders is photosensitized (made hydrophobic) for attracting ink to dots positioned for applying a single color, or the appearance of a single color, on the final printing product. Typically, three primary colors are applied in a three-color printing press and black ink applied at a fourth printing head. In order to provide a clear, blended color picture, the registration or the positioning of each set of differently colored dots superimposed on the same sheet paper becomes a critical feature and requires each plate and blanket cylinder to be the same diameter, all sets to be synchronously geared to the impression cylinder and rolling contact pressure against the impression cylinder and a makeready position (relaxed pressure, non-contact). It also requires careful adjustment of rotation axial and skew orientation and appropriate contact pressure between the plate and blanket cylinders, as well as between the blanket cylinder with the impression cylinder at each color station or each color head.
Moreover, the plate cylinder relative to the blanket cylinder and the pair of plate and blanket cylinders relative to the impression cylinders, cannot be maintained at a fixed position but, rather, must be moveable relative to each other between an impression position in order to appropriately mount new plates onto the plate cylinders, new blankets on the blanket cylinders and also, to accommodate the thickness of the plates and, further, to appropriately provide an amount of pressure according to the number or density of color dots required for a given color strength and for a particular printing job.
The plate and blanket cylinder are designed to be of equal diameter and the blanket cylinder is sometimes provided with a corresponding groove across its width. On impression, the plate cylinder and blanket cylinder are brought into rolling contact under pressure and the blanket cylinder is brought into rolling contact under impression pressure against the impression cylinder. Even where the bearings and axles are made with a close tolerances and high strength, the amount of pressure across the entire surface of the plate and blanket cylinder is substantial. The rolling contact pressure is often adjusted in terms of the width or thickness of a "stripe" or line across the surface of the contacting cylinders. During makeready, impression pressure is applied with the press stopped, so that the ink and dampening fluid is completely flattened or squeezed off, and it appears to be absent in an area of a stripe across the surface of the cylinders. A thinner stripe corresponds to lighter pressure and a thicker stripe indicates more pressure. When the plate gap rolls past the blanket cylinder, or rolls into a rotary position adjacent to a corresponding blanket gap in the blanket cylinder, the two cylinders, no longer push against each other and they tend to move toward each other slightly, reducing the pressure on the impression cylinder and causing a light print area or a streak. When the cylinders rotate to the other edge of the gap, the cylinders then must move quickly away from each other as the surface-to-surface rolling contact is reinitiated. This can cause a vibration and also can cause a corresponding bounce or chatter, slurring or skipping due to a reduction in pressure and reinitiation of pressure between the blanket cylinder and the impression cylinder. When this change in rolling contact pressure is great, streaking, skipping, bouncing or chattering can adversely and noticeably affect the quality of the print. Even when the change in impression pressure is small, it can less perceivably affect the accuracy of registration of the sets of image-forming dots so that a lack of clarity and precision of the total image can result. This is sometimes referred to as "slurring." If one color head does not continuously and properly align with the sets of dots for another color head, the image will not be sharp, the color blend will not be true or the image may be blurred or slurred.
In the past, to correct this and to reduce the movement of the plate cylinder with respect to the blanket cylinder and the corresponding change in impression pressure, some press manufacturers have produced circular plates rigidly affixed at either end of the plate and blanket cylinder. The circular plates are machined with a close tolerance and are accurately mounted concentric with the cylinder surfaces to provide a continuous rolling contact. While this prior system avoids the chattering problem, it creates its own problem in that the amount of pressure between the plate and blanket cylinder cannot be easily adjusted from one printing job to the next or cannot easily accommodate printing plates of even slightly different thickness. An experienced press operator can adjust the contact with this type of direct rolling contact end plates by wrapping the plate cylinder with a thin shim sheet of paper prior to attaching the photosensitized printing plate. This takes a substantial amount of time and it is not usually warranted for short runs. Even in instances where the press operator takes the time to remove the plate and install different thicknesses of shim sheets until the desired pressure between the two plates is obtained, Also, it will be seen when the plate cylinder is packed, the blanket will usually also be packed to keep the cylinders the same size. The blanket cylinder is typically gear-driven from the impression cylinder and the plate cylinder is gear-driven from the blanket cylinder. If their rolling diameters are not identical, then the one-to-one rolling contact intended by the gear drive is not necessarily achieved and slippage or slight sliding and blurring between the surface can result.
The adjustment of the pressure between the plate cylinder and the blanket cylinder is often referred to as adjusting the stripe thickness or width. Under pressure the line of contact or flattened ink between one cylinder surface and the other cylinder surface may be wider or narrower, depending on how much pressure there is between the two cylinders. This is often, then, referred to as "adjusting the stripe." A stripe in a range of 1/8" to about 3/8" might be acceptable, depending upon the press and printing job. An experienced press operator can appropriately adjust the stripe width, depending upon factors such as the density of printing of a particular color, the thickness or viscosity of the ink being used and the amount of dampening fluid applied. The stripe is generously proportional to the contacting force or rolling pressure and should normally be the same from one end of the cylinders to the other end.
It can be important to maintain proper printing and registration to adjust the stripe width, yet it is also desirable to avoid the change in pressure caused when each plate connection channel rolls unopposed past the blanket cylinder.